Rhai documentation: remove incorrect statement about request.subgraph…

… fields (#3808)

It is possible to modify `request.subgraph` fields from a `Rhai` script
and so the documentation should reflect that.

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- [ ] Changes are compatible[^1]
- [ ] Documentation[^2] completed
- [ ] Performance impact assessed and acceptable
- Tests added and passing[^3]
    - [ ] Unit Tests
    - [ ] Integration Tests
    - [ ] Manual Tests

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.changesets/docs_garypen_fix_rhai_subgraph_docs.md

@@ -0,0 +1,5 @@
+### Rhai documentation: remove incorrect statement about request.subgraph fields ([PR #3808](https://github.com/apollographql/router/pull/3808))
+
+It is possible to modify `request.subgraph` fields from a `Rhai` script and so the documentation should reflect that.
+
+By [@garypen](https://github.com/garypen) in https://github.com/apollographql/router/pull/3808

docs/source/customizations/rhai-api.mdx

@@ -289,7 +289,7 @@ Note: get() may fail, so it's best to handle exceptions when using it.
 
 ## `Request` interface
 
-All callback functions registered via `map_request` are passed a `request` object that represents the request sent by the client. This object provides the following fields, most of which a callback can modify in-place (read-write):
+All callback functions registered via `map_request` are passed a `request` object that represents the request sent by the client. This object provides the following fields:
 
 ```
 request.context
@@ -302,9 +302,9 @@ request.body.extensions
 request.uri.host
 request.uri.path
 ```
-Note: `method` is read-only.
+Note: These fields are typically modifiable, apart from `method` which is always read-only. However, when the callback service is `subgraph_service`,the only modifiable field is `request.context`.
 
-**For `subgraph_service` callbacks only,** the `request` object provides _additional_ fields for interacting with the request that will be sent to the corresponding subgraph:
+**For `subgraph_service` callbacks only,** the `request` object provides _additional_ modifiable fields for interacting with the request that will be sent to the corresponding subgraph:
 
 ```
 request.subgraph.headers
@@ -316,8 +316,6 @@ request.subgraph.uri.host
 request.subgraph.uri.path
 ```
 
-These additional fields are shared across all subgraph invocations and are thus read-only.
-
 ### `request.context`
 
 The context is a generic key/value store that exists for the entire lifespan of a particular client request. You can use this to share information between multiple callbacks throughout the request's lifespan.